One-wire measuring devices may be used especially in the field of monitoring vital signs and parameters from biopotentials (e.g. ECG) or/and from impedance measurements (e.g. respiration).
Biopotentials provide information on some electrical physiological process of the human or animal body. Electrodes are used at the interface of the measuring device and the body. Usually, the electrodes are applied on the skin, but they can be elsewhere. They can also be needle electrodes inserted in the body.
The interface between the body and the electrodes is generally a chemical cell that translates ionic current into metallic current. A gel is usually used between the electrodes and the body in order to decrease the contact impedance and consequently the noise and motion artefacts. However, some measuring devices use dry electrodes. Furthermore, some electrodes are totally isolated and only displacement (AC) currents are picked up.
Typical biopotentials are ECG (or EKG), i.e., electrocardiography. Others include for instance EEG (electroencephalography), EOG (electro-oculography), EMG (electromyography), etc. FIG. 1 shows a 12-lead Mason-Likar ECG system. The left side of the figure indicates the placement of the adhesive electrodes and the right side shows a typical schematic diagram of the front-end electronic circuitry used to serve the affixed electrodes.
Electronic circuitry is usually located separately from the electrodes: either in a small recorder at the belt (Holter's system) or in a bed-side box. However, especially for EEG (weak signal) or for isolated electrodes (very high impedance), part of the electronic system may be located at the electrodes in order to keep the noise as low as possible. Such electrodes are called ‘active electrodes’.
In FIG. 1, the electrode potentials are buffered by operational amplifiers connected as followers, i.e., with unitary gains. These followers give high input impedance and low output impedance.
The electrode cables are shielded. The shield is optimal when driven by the output voltage of the follower. On the one hand, this protects from capacitive coupling of disturbances, and on the other hand, if the gain g of the follower is exactly unity, it limits the input impedance to the amplifier input impedance. In the real world, the CMRR (common mode rejection ratio) of the follower operational amplifier is not infinite, which means that g is not exactly unity. This results in incomplete cancellation of the parasitic shunt capacitance Cp between the electrode wire and its shield; the capacitance is actually reduced to Cp(1−g), i.e., equal to 0 only if g is exactly unity.
The potential measured at the three electrodes R, L, and F is averaged to the so-called Wilson's central terminal W by three resistances. This potential is set equal to the electronics ground (i.e. to 0V) using a feedback filter and a so-called guard drive electrode G (sometimes also called ‘right leg drive electrode’). The transfer function of a typical feedback filter is an integrator with a sign inversion, i.e., −1/ST, where S is the Laplace variable and T the time constant corresponding to the closed-loop frequency. The other electrodes, i.e., V1 to V6, measure, with respect to the Wilson terminal W, the so-called precordial leads.
The input impedance of the measuring device through electrode G is very low and allows the mains disturbance current—originating from capacitive coupling between the body and the measuring device—to preferably flow through electrode G rather than through the measurement electrodes. This way, the mains influence on the biopotential measurement is minimized.
The ECG electrodes are usually disposable, adhesive gel electrodes applied directly at the appropriate place by a doctor or a trained nurse. The electrodes are then connected to wires (one different wire for each electrode). The wires are connected to a recorder, which is a small electronic unit usually placed at the belt. The wires are sometimes attached to the body with tape straps in a way chosen by the doctor or the trained nurse, so as to minimize the risk that the wire weight displaces an electrode and to maximize the subject comfort.
Electrodes are also used in impedance measurement, such as for instance impedance plethysmography, impedance cardiography, body composition impedance, impedance tomography, skin impedance, etc. Impedances are usually measured at high frequency (relative to body frequencies), typically 50 kHz. In order to separate the high impedance at the electrode/body interface from the low impedance of the inner body tissues, a 4-wire scheme such as depicted in FIG. 2 is usually used. In this figure, the impedance Z which is to be measured, and the four interface impedances are drawn in hashed line. Two electrodes are connected to a current source i, while two other electrodes for voltage measurement (V2−V1) are connected to the front-end amplifiers.
To limit as much as possible the discomfort brought by any measuring device, it is advantageous to reduce as far as possible the number of interfaces with the body. In particular, it is of interest to share electrodes for biopotential and impedance measurements whenever this is possible. Furthermore, the guard drive electrode, the electronics unit and the cables can be justified for technical reasons, but are a significant source of obtrusiveness and discomfort for the subject.
Objective non-supervised assessment of cough remains a main challenge in long-term ambulatory data-collection systems.
Standard developments relay either on the digital recording of cough sounds, the analysis of chest wall EMG signals or a combination of both (Smith, “Ambulatory methods for recording cough”, Pulmonary Pharmacology & therapeutics 20 (2007) 313-318). Methods based on the analysis of the cough acoustic signal have been reported to provide sensitivity values of 82% (true positive detections over true positive detections and false negative detections). The main reason for such low performances is the inter-subject variability of cough sounds acoustic properties as well as the dependency to transducer placement and configuration. Ambient noise might be an additional important source of false positive detections. (Matos et al., “Detection of cough signals in continuous audio recordings using Hidden Markov Models”, IEEE Transactions Biomedical Engineerings, 2006; 53:1078-83).
Among some new developments appeared in the last few years, one must cite the efforts recently performed with the Vivometric Lifeshirt (Coyle et al., “Evaluation of an ambulatory system for the quantification of cough frequency in patients with chronic obtrusive pulmonary disease”, Cough 2005). The authors used the LifeShirt to record Respiratory Inductance Plethysmography (RIP), acoustic sounds, electrocardiogram and accelerometry and reported sensitivity values of 97% (Smith, “Cough: assessment and equipment”, The Buyers Guide to Respiratory Care Products, 2007). The detailed description of the method is given in a Vivometric's patent (Coyle et al. “Systems and methods for monitoring cough”, US200710276278).
The main purpose of the invention is to propose a front-end electronic circuit for biopotential and impedance measurements with outstanding performances (very high input impedance and gain very close to unity). In a preferred embodiment, the explicit guard electrode and the explicit electronic unit at the belt are no longer necessary; all electronics is embedded in units placed directly at the measurement sites. Moreover, the proposed front-end electronic circuit allows a drastic simplification of the cabling and connectors since all units are connected to only one wire (the theoretical minimum) for potential reference and current return. In a preferred embodiment, this wire does not even require an electrical isolation and can be embedded in the textile of a shirt, in a garment, mesh, belt, etc.